Integrated circuits with cascode transistor

ABSTRACT

An integrated circuit includes an analog circuit including a first transistor. At least one cascode transistor is electrically coupled with the first transistor in a series fashion. A drain and a gate of the cascode transistor are electrically coupled with a gate of the first transistor of the analog circuit. The at least one cascode transistor is operated in a saturation mode or a sub-threshold mode.

TECHNICAL FIELD

The present disclosure relates generally to the field of semiconductorintegrated circuits, and more particularly, to integrated circuits withcascode transistors.

BACKGROUND

The semiconductor industry has experienced continual rapid growth due tocontinuous improvements in the integration density of various electroniccomponents (i.e., transistors, diodes, resistors, capacitors, etc.). Forthe most part, this improvement in integration density has come fromrepeated reductions in minimum feature size, allowing for theintegration of more components into a given area. The scale down offeatures of integrated circuits has been implemented in variousapplications, e.g., digital circuits, analog circuits, and mixed-signalcircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1A is a schematic drawing of an exemplary integrated circuitincluding an N-type cascode transistor.

FIG. 1B is a schematic drawing of an exemplary integrated circuitincluding a P-type cascode transistor.

FIG. 2A is a schematic drawing of an integrated circuit including a biascircuit.

FIG. 2B is a schematic drawing of another exemplary integrated circuitincluding a bias circuit.

FIG. 3 is a schematic drawing of an exemplary operational amplifier.

FIG. 4 is a schematic drawing of an exemplary constant gm bias circuit.

FIG. 5 is a schematic drawing of an exemplary bandgap reference circuit.

DETAILED DESCRIPTION

In analog circuits, cascode technology is deployed to enhance electricalperformances. In cascode technology, a cascode transistor iselectrically coupled with a transistor in series. As the cascodetransistor is operated as a diode having a low input resistance to thetransistor, the voltage gain of the transistor becomes small, such thatthe bandwidth of the analog circuit is increased.

One cascode technique is to electrically connect a gate and a drain ofthe cascode transistor together. A gate and a drain of the transistorare electrically connected to each other. The gate of the cascodetransistor is electrically isolated from the gate of the transistor.During the operation, the cascode transistor and the transistor are bothoperated in a saturation mode. In such a configuration, the supplyvoltage is designed to be larger than a sum of threshold voltages of thecascode transistor and the transistor. This configuration of the cascodetransistor and the transistor makes the voltage headroom small.

Another cascode technique is to supply a bias voltage to the gate of thecascode transistor to enhance the voltage headroom. To provide a desiredbias voltage, the bias circuit electrically coupled with the gate of thecascode transistor is complicated and may be difficult to design.

It is understood that the following disclosure provides many differentembodiments, or examples, for implementing different features of variousembodiments. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Forexample, the formation of a first feature over or on a second feature inthe description that follows may include embodiments in which the firstand second features are formed in direct contact, and may also includeembodiments in which additional features may be formed between the firstand second features, such that the first and second features may not bein direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition of reference numerals indicates the same element contained indifferent embodiments and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

FIG. 1A is a schematic drawing of an exemplary integrated circuitincluding an N-type cascode transistor. In FIG. 1A, an integratedcircuit 100 includes an analog circuit 110. In some embodiments, theanalog circuit 110 is an operational amplifier, a constant-gm biascircuit, a bandgap reference circuit, a current mirror, an analog todigital converter (ADC), a digital to analog converter (DAC), a pumpcircuit, a multiplexer, or one of other analog circuitries.

Referring to FIG. 1A, the analog circuit 110 includes a transistor M₁.The transistor M₁ is an N-type transistor as shown in FIG. 1A. Theintegrated circuit 100 includes at least one cascode transistor, e.g.,cascode transistor M_(C1) that is electrically coupled with thetransistor M₁ in a series fashion. A drain and a gate of the cascodetransistor M_(C1) are electrically coupled with a gate of the transistorM₁ of the analog circuit 110. In some embodiments, a source and a bulkof the transistor M₁ are electrically coupled with a bulk of the cascodetransistor M_(C1) and electrically grounded.

During an operation, the cascode transistor M_(C1) is operated in asaturation mode or a sub-threshold mode. The transistor M₁ is operatedin a saturation mode. By operating the cascode transistor M₁ in asaturation mode or a sub-threshold mode, the voltage headroom betweenthe drain of the cascode transistor M₁ and a supply voltage becomeslarger. Additionally, the gates of the cascode transistor M_(C1) and thetransistor M₁ are electrically coupled with the drain of the cascodetransistor M₁. No bias circuit is electrically coupled to a bulk of thecascode transistor M_(C1) and configured to provide a bias to adjust theoperation mode of the cascode transistor M_(C1). As a result, the designof the integrated circuit 100 is simplified. The area that is assignedto accommodate the bias circuit is saved.

In some embodiments, the cascode transistor M_(C1) is operated in asub-threshold mode, and the transistor M₁ is operated in a saturationmode. To achieve the operation, the voltage differential (V_(GSC1))between the gate and the source of the cascode transistor M_(C1) is lessthan the threshold voltage (V_(THC1)) of the cascode transistor M_(C1).The threshold voltage (V_(THC1)) of the cascode transistor M_(C1) isless than the threshold voltage (V_(TH1)) of the transistor M₁. Therelationship among the voltage differential V_(GSC1) and the thresholdvoltages V_(THC1) and V_(TH1) can be shown as below:

V_(GSC1)V_(THC1)<V_(TH1)

In some embodiments, the threshold voltage (V_(THC1)) of the cascodetransistor M_(C1) being less than the threshold voltage (V_(TH1)) of thetransistor M₁ allows a channel length of the cascode transistor M_(C1)to be designed shorter than a channel length of the transistor M₁. Insome embodiments, a channel width of the cascode transistor M_(C1) isdesigned to be wider than a channel width of the transistor M₁. In otherembodiments, the cascode transistor M_(C1) is a native transistor andthe transistor M₁ is not a native transistor. For example, the thresholdvoltage (V_(THC1)) of the cascode transistor M_(C1) is around 0, and thethreshold voltage (V_(TH1)) of the transistor M₁ is higher than 0. Instill other embodiments, the cascode transistor M₁ is a low thresholdvoltage (LVT) transistor or an ultra-low threshold voltage (ULVT)transistor. For example, the cascode transistor M_(C1) is an N-type LVTcascode transistor. The P-type dopant concentration in the channelregion of the cascode transistor is lower than that of the transistor.

FIG. 1B is a schematic drawing of an exemplary integrated circuitincluding a P-type cascode transistor. In FIG. 1B, the integratedcircuit 100 includes an analog circuit 120. In some embodiments, theanalog circuit 120 is an operational amplifier, a constant-gm biascircuit, a bandgap reference circuit, a current mirror, an analog todigital converter (ADC), a digital to analog converter (DAC), a pumpcircuit, a multiplexer, or one of other analog circuitries.

Referring to FIG. 1B, the analog circuit 120 includes a transistor M₂.The transistor M₂ is a P-type transistor as shown in FIG. 1B. Theintegrated circuit 100 includes at least one cascode transistor, e.g.,cascode transistor M_(C2) that is electrically coupled with thetransistor M₂ in a series fashion. The cascode transistor M_(C2) is aP-type cascode transistor. A drain and a gate of the cascode transistorM_(C2) are electrically coupled with a gate of the transistor M₂ of theanalog circuit 120. In some embodiments, a source and a bulk of thetransistor M₂ are electrically coupled to each other and with a supplyvoltage V_(DD).

As noted, the cascode transistor M_(C2) is operated in a saturation modeor a sub-threshold mode. The transistor M₂ is operated in a saturationmode. By operating the cascode transistor M_(C2) in a saturation mode ora sub-threshold mode, the voltage headroom of the analog circuit 120becomes larger. Additionally, the gates of the cascode transistor M_(C2)and the transistor M₂ are electrically coupled with the drain of thecascode transistor M_(C2). No bias circuit is electrically coupled withand used to provide a bias to the bulk of the cascode transistor M_(C2)to adjust the operation mode of the cascode transistor M_(C2).

In some embodiments, the cascode transistor M_(C2) is operated in asub-threshold mode, and the transistor M₂ is operated in a saturationmode. To achieve the operation, the absolute value of the voltagedifferential (V_(GSC2)) between the gate and the source of the cascodetransistor M_(C2) is less than the absolute value of the thresholdvoltage (V_(THC2)) of the cascode transistor M_(C2). The absolute valueof the threshold voltage (V_(THC2)) of the cascode transistor M_(C2) isless than the absolute value of the threshold voltage (V_(TH2)) of thetransistor M₂. The relationship among the absolute values of the voltagedifferential V_(GSC2) and the threshold voltages V_(THC2) and V_(TH2) isshown as below:

|V

₁ GSC2|<V ₁ THC2|<|V ₁ TH2|

In some embodiments, the absolute value of the threshold voltage(V_(THC2)) of the cascode transistor M_(C2) being less than the absolutevalue of the threshold voltage (V_(TH2)) of the transistor M₂ allows achannel length of the cascode transistor M_(C2) to be designed shorterthan a channel length of the transistor M₂. In some embodiments, achannel width of the cascode transistor M_(C2) is designed to be widerthan a channel width of the transistor M₂. In some embodiments, thecascode transistor M_(C2) is a native transistor and the transistor M₂is not a native transistor. For example, the absolute value of thethreshold voltage (V_(THC2)) of the cascode transistor M_(C2) is around0, and the absolute value of the threshold voltage (V_(TH2)) of thetransistor M₂ is higher than 0. In still some embodiments, the cascodetransistor M_(C2) is a low threshold voltage (LVT) transistor or anultra-low threshold voltage (ULVT) transistor. As noted, the cascodetransistor M_(C2) is a P-type LVT transistor. The N-type dopantconcentration in the channel region of the cascode transistor is lowerthan that of the transistor.

FIG. 2A is a schematic drawing of an integrated circuit including a biascircuit. In FIG. 2A, an integrated circuit 200 includes an analogcircuit 210, which is the same as or similar to the analog circuit 110described above in conjunction with FIG. 1A. In FIG. 2A, the analogcircuit 210 includes a transistor M₁ that is the same as or similar tothe transistor M₁ described above in conjunction with FIG. 1A. Theintegrated circuit 200 includes at least one cascode transistor, e.g.,cascode transistor M_(C3) that is electrically coupled with thetransistor M₁ in a series fashion. A drain and a gate of the cascodetransistor M_(C3) are electrically coupled with a gate of the transistorM₁ of the analog circuit 210.

Referring to FIG. 2A, the integrated circuit 210 includes a bias circuit230. The bias circuit 230 is electrically coupled with a bulk or a wellregion of the cascode transistor M_(C3). For example, the cascodetransistor M_(C3) is an N-type cascode transistor. The bias circuit 230is electrically coupled with a P-type well region or a P-type substrateof the cascode transistor M_(C3).

As noted, the cascode transistor M_(C3) is operated in a saturation modeor a sub-threshold mode. The transistor M₁ is operated in a saturationmode. To achieve the operation, the threshold voltage (V_(THC3)) of thecascode transistor M_(C3) is lower than the threshold voltage (V_(TH1))of the cascode transistor M₁.

In FIG. 2A, the bias circuit 230 is configured to lower the thresholdvoltage (V_(THC3)) of the cascode transistor M_(C3). For example, thetransistor M₁ and the cascode transistor M_(C3) are designed with thesimilar channel length, channel width, and/or dopant concentration. Thebias circuit 230 is configured to provide a voltage to the P-type wellregion of the cascode transistor M_(C3), such that the threshold voltage(V_(THC3)) of the cascode transistor M_(C3) is lower than the thresholdvoltage (V_(TH1)) of the cascode transistor M₁.

In some embodiments, the bias circuit 230 includes transistors M₃ and M₄that are electrically coupled to each other in a series fashion. Thetransistor M₃ is an N-type transistor and the transistor M₄ is a P-typetransistor. A gate and a drain of the transistor M₃ are electricallycoupled with a node N₁ between the transistors M₃ and M₄. The node N₁ iselectrically coupled with a bulk or a well region of the cascodetransistor M_(C3). A bulk and a source of the transistor M₄ areelectrically coupled to each other and with a supply voltage V_(DD). Agate of the transistor M₄ is controlled by a bias voltage that isprovided from a bias circuit (not shown).

FIG. 2B is a schematic drawing of another exemplary integrated circuitincluding a bias circuit. In FIG. 2B, an integrated circuit 200 includesan analog circuit 220, which is the same as or similar to the analogcircuit 120 described above in conjunction with FIG. 1B. In FIG. 2B, theanalog circuit 220 includes a transistor M₂ that is the same as orsimilar to the transistor M₂ described above in conjunction with FIG.1B. The integrated circuit 200 includes at least one cascode transistor,e.g., cascode transistor M_(C4) that is electrically coupled with thetransistor M₂ in a series fashion. A drain and a gate of the cascodetransistor M_(C4) are electrically coupled with a gate of the transistorM₂ of the analog circuit 220.

Referring to FIG. 2B, the integrated circuit 220 includes a bias circuit240. The bias circuit 240 is electrically coupled with a well region ofthe cascode transistor M_(C4). For example, the cascode transistorM_(C4) is a P-type cascode transistor. The bias circuit 240 iselectrically coupled with an N-type well region of the cascodetransistor M_(C4).

As noted, the cascode transistor M_(C4) is operated in a saturation modeor a sub-threshold mode. The transistor M₂ is operated in a saturationmode. It can be achieved by controlling the threshold voltage (V_(THC4))of the cascode transistor M_(C4) lower than the threshold voltage(V_(TH2)) of the cascode transistor M₂.

In FIG. 2B, the bias circuit 240 is configured to lower the thresholdvoltage (V_(THC4)) of the cascode transistor M_(C4). For example, thetransistor M₂ and the cascode transistor M_(C4) have the similar channellength, channel width, and dopant concentration. The bias circuit 240 isconfigured to provide a voltage to the N-type well region of the cascodetransistor M_(C4), such that the threshold voltage (V_(THC4)) of thecascode transistor M_(C4) is lower than the threshold voltage (V_(TH2))of the cascode transistor M₂.

In some embodiments, the bias circuit 240 includes transistors M₅ and M₆that are electrically coupled to each other in a series fashion. Thetransistors M₅ and M₆ are N-type transistors. A gate and a drain of thetransistor M₆ are electrically coupled to each other. A node N₂ betweenthe transistors M₅ and M₆ is electrically coupled with the N-type wellregion of the cascode transistor M_(C4). In some embodiments, a bulk anda source of the transistor M₅ are electrically coupled with a bulk ofthe transistor M₆. A gate of the transistor M₅ is controlled by a biasvoltage that is provided by a bias circuit (not shown).

Following are various exemplary analog circuits that include at leastone cascode transistor described above in conjunction with FIGS. 1A-1Band 2A-2B. It is noted that the applications of the cascode technologyof this application are not limited thereto. It is also noted that theconfigurations of the analog circuits below are merely exemplary. Thescope of this application is not limited thereto.

FIG. 3 is a schematic drawing of an exemplary operational amplifier. InFIG. 3, an operational amplifier 300 includes transistors M₃₀₁-M₃₀₆ andcascode transistors M_(C301)-M_(C306). The transistors M₃₀₁ and M₃₀₃ andthe cascode transistors M_(C301) and M_(C303) are electrically coupledto each other in a series fashion between a current source and a supplyvoltage V_(DD). The transistors M₃₀₂ and M₃₀₄ and the cascodetransistors M_(C302) and M_(C304) are electrically coupled to each otherin a series fashion between the current source and the supply voltageV_(DD). The transistors M₃₀₅ and M₃₀₆ and the cascode transistorsM_(C305) and M_(C306) are electrically coupled to each other in a seriesfashion. Gates of the transistors M₃₀₁ and M₃₀₂ of the operationalamplifier 300 are configured to receive input signals. The operationalamplifier 300 outputs at V_(OUT).

In some embodiments, gates of the cascode transistors M_(C301),M_(C302), and M_(C305) and the transistor M₃₀₅ are electrically coupledwith each other. Gates of the cascode transistors M_(C303), M_(C304),and M_(C306) and gates of the transistors M₃₀₃, M₃₀₄, and M₃₀₆ areelectrically coupled with each other. Bulks of the transistors M₃₀₁ andM₃₀₂ and the bulk of the cascode transistor M_(C302) are electricallycoupled with the current source. Bulks of the transistors M₃₀₃ and M₃₀₄are electrically coupled with the supply voltage V_(DD). A source and abulk of the transistor M₃₀₆ are electrically coupled with each other. Asource and a bulk of the transistor M₃₀₅ are electrically coupled witheach other and electrically grounded.

Referring to FIG. 3, the configuration and operation of the transistorsM₃₀₁, M₃₀₂ and M₃₀₅ are the same as or similar to those of thetransistor M₁ described above in conjunction with FIG. 1A. Theconfiguration and operation of the transistors M₃₀₃, M₃₀₄ and M₃₀₆ arethe same as or similar to those of the transistor M₂ described above inconjunction with FIG. 1B. The configuration and operation of the cascodetransistor M_(C305) are the same or similar to those of the cascode M₁described above in conjunction with FIG. 1A. The configuration andoperation of the cascode transistor M_(C304) are the same or similar tothose of the cascode M_(C2) described above in conjunction with FIG. 1B.

In FIG. 3, configurations of the cascode transistors M_(C301) andM_(C302) are different from the cascode transistor M_(C305). Forexample, gates of the cascode transistors M_(C301) and M_(C302) are notelectrically coupled with the gates of the transistors M₃₀₁ and M₃₀₂,respectively. The cascode transistors M_(C303) and M_(C306) aredifferent from the cascode transistor M_(C304). For example, drains ofthe cascode transistors M_(C303) and M_(C306) are not electricallycoupled with the gates of the cascode transistors M_(C303) and M_(C306),respectively. During the operation, the cascode transistors M_(C301),M_(C302), M_(C303) and M_(C306) are selectively operated in a saturationmode or a sub-threshold mode.

FIG. 4 is a schematic drawing of an exemplary constant gm bias circuit.In FIG. 4, a constant gm bias circuit 400 includes transistors M₄₀₁-M₄₀₄and cascode transistors M_(C401)-M_(C404). The transistors M₄₀₁ and M₄₀₃and the cascode transistors M_(C401) and M_(C403) are electricallycoupled to each other in a series fashion between ground and a supplyvoltage V_(DD). The transistors M₄₀₂ and M₄₀₄ and the cascodetransistors M_(C402) and M_(C404) are electrically coupled to each otherin a series fashion between a resistor R₄₀₁ and the supply voltageV_(DD).

In some embodiments, gates of the cascode transistors M_(C401) andM_(C402) and gates of the transistors M₄₀₁ and M₄₀₂ are electricallycoupled with each other. Gates of the cascode transistors M_(C403) andM_(C404) and gates of the transistors M₄₀₃ and M₄₀₄ are electricallycoupled with each other. A bulk and a source of the transistor M₄₀₁ areelectrically coupled to each other and electrically grounded. A bulk anda source of the transistor M₄₀₂ are electrically coupled to each otherand with the resistor R₄₀₁. Bulks of the transistors M₄₀₃ and M₄₀₄ areelectrically coupled with the supply voltage V_(DD).

Referring to FIG. 4, the transistors M₄₀₁ and M₄₀₂ each are the same asor similar to the transistor M₁ described above in conjunction with FIG.1A. The transistors M₄₀₃ and M₄₀₄ each are the same as or similar to thetransistor M₂ described above in conjunction with FIG. 1B. Theconfiguration and operation of the cascode transistors M_(C401) andM_(C402) are the same or similar to those of the cascode described abovein conjunction with FIG. 1A. The configuration and operation of thecascode transistors M_(C403) and M_(C404) each are the same or similarto those of the cascode M_(C2) described above in conjunction with FIG.1B.

FIG. 5 is a schematic drawing of an exemplary bandgap reference circuit.In FIG. 5, a bandgap reference circuit 500 includes transistorsM₅₀₁-M₅₀₄, cascode transistors M_(C501)-M_(C504), resistors R₅₀₁-R₅₀₃,and bipolar transistors B₅₀₁-B₅₀₂. The transistors M₅₀₁ and M₅₀₃, thecascode transistors M_(C501) and M_(C503), the resistor R₅₀₂, and thebipolar transistor B₅₀₂ are electrically coupled to each other in aseries fashion between ground and a supply voltage V_(DD). Thetransistors M₅₀₂ and M₅₀₄, the cascode transistors M_(C502) andM_(C504), the resistors R₅₀₁ and R₅₀₃, and the bipolar transistor B₅₀₁are electrically coupled to each other in a series fashion betweenground and the supply voltage V_(DD).

In some embodiments, gates of the cascode transistors M_(C501) andM_(C502) and gates of the transistors M₅₀₁ and M₅₀₂ are electricallycoupled with each other. Gates of the cascode transistors M_(C503) andM_(C504) and gates of the transistors M₅₀₃ and M₅₀₄ are electricallycoupled with each other. A bulk and a source of the transistor M₅₀₁ areelectrically coupled to each other and with the output V_(OUT) of thebandgap reference circuit 500. A bulk and a source of the transistorM₅₀₂ are electrically coupled to each other and with the resistor R₅₀₃.Bulks of the transistors M₅₀₃ and M₅₀₄ are electrically coupled with thesupply voltage V_(DD).

Referring to FIG. 5, the transistors M₅₀₁ and M₅₀₂ each are the same asor similar to the transistor M₁ described above in conjunction with FIG.1A. The transistors M₅₀₃ and M₅₀₄ each are the same as or similar to thetransistor M₂ described above in conjunction with FIG. 1B. Theconfiguration and operation of the cascode transistors M_(C501) andM_(C502) are the same or similar to those of the cascode described abovein conjunction with FIG. 1A. The configuration and operation of thecascode transistors M_(C503) and M_(C504) are the same or similar tothose of the cascode M_(C2) described above in conjunction with FIG. 1B.

As noted, the descriptions of the analog circuits in conjunction withFIGS. 3-5 are merely exemplary. In some embodiments, the bias circuits230 and 240 described above in conjunction with FIGS. 2A and 2B areselectively adapted to control the threshold voltages of the cascodetransistors described above in conjunction with FIGS. 3-5.

In some exemplary embodiments of this application, an integrated circuitincludes an analog circuit including a first transistor. At least onecascode transistor is electrically coupled with the first transistor ina series fashion. A drain and a gate of the cascode transistor areelectrically coupled with a gate of the first transistor of the analogcircuit. The at least one cascode transistor is operated in a saturationmode or a sub-threshold mode.

In some exemplary embodiments of the current application, an integratedcircuit includes an analog circuit including a first transistor. Atleast one cascode transistor is electrically coupled with the firsttransistor in a series fashion. A drain and a gate of the cascodetransistor are electrically coupled with a gate of the first transistorof the analog circuit. The at least one cascode transistor includes atleast one of a native transistor, a low threshold voltage (LVT)transistor, and an ultra-low threshold voltage (ULVT) transistor.

In some exemplary embodiments of the application, an integrated circuitincludes a first P-type cascode transistor electrically coupled with afirst P-type transistor in a series fashion. A second P-type cascodetransistor is electrically coupled with a second P-type transistor in aseries fashion. Gates of the first P-type cascode transistor, the secondP-type cascode transistor, the first P-type transistor, and the secondP-type transistor are electrically coupled to each other and with adrain of the first P-type cascode transistor.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. An integrated circuit comprising: an analogcircuit including a first transistor; and at least one cascodetransistor electrically coupled with the first transistor in a seriesfashion, wherein a drain and a gate of the cascode transistor areelectrically coupled with a gate of the first transistor of the analogcircuit, and the at least one cascode transistor is operated in asaturation mode or a sub-threshold mode.
 2. The integrated circuit ofclaim 1, wherein a threshold voltage of the at least one cascodetransistor is lower than a threshold voltage of the first transistor. 3.The integrated circuit of claim 2, wherein a channel length of the atleast one cascode transistor is shorter than a channel length of thefirst transistor.
 4. The integrated circuit of claim 2, wherein achannel width of the at least one cascode transistor is wider than achannel width of the first transistor.
 5. The integrated circuit ofclaim 2, wherein the at least one cascode transistor includes at leastone native transistor and the first transistor is not a nativetransistor.
 6. The integrated circuit of claim 2, wherein a dopantconcentration in a channel region of the at least one cascode transistoris lower than that of the first transistor.
 7. The integrated circuit ofclaim 1 further comprising: a bias circuit electrically coupled with awell region of the at least one cascode transistor, wherein the biascircuit is configured to lower a threshold voltage of the at least onecascode transistor.
 8. The integrated circuit of claim 7, wherein the atleast one cascode transistor includes at least one N-type cascodetransistor and the bias circuit comprises: a second transistor and athird transistor electrically coupled to each other in a series fashion,wherein a gate and a drain of the third transistor are electricallycoupled with a node between the second and third transistors, and thenode between the second and third transistors is electrically coupledwith the well region of the at least one N-type cascode transistor. 9.The integrated circuit of claim 1, wherein the at least one cascodetransistor includes at least one P-type cascode transistor and the biascircuit comprises: a second transistor and a third transistorelectrically coupled to each other in a series fashion, wherein a gateand a drain of the second transistor are electrically coupled to eachother, and a node between the second and third transistors iselectrically coupled with a well region of the at least one P-typecascode transistor.
 10. The integrated circuit of claim 1, wherein theanalog circuit is an operational amplifier, a constant-gm bias circuit,a pump circuit, a converter, a multiplexer, or a bandgap referencecircuit.
 11. An integrated circuit comprising: an analog circuitincluding a first transistor; and at least one cascode transistorelectrically coupled with the first transistor in a series fashion,wherein a drain and a gate of the cascode transistor are electricallycoupled with a gate of the first transistor of the analog circuit, andthe at least one cascode transistor includes at least one of a nativetransistor, a low threshold voltage (LVT) transistor, and an ultra-lowthreshold voltage (ULVT) transistor.
 12. The integrated circuit of claim11, wherein the at least one cascode transistor is operated in asaturation mode or a sub-threshold mode.
 13. The integrated circuit ofclaim 11 further comprising: a bias circuit electrically coupled with awell region of the at least one cascode transistor, wherein the biascircuit is configured to lower a threshold voltage of the at least onecascode transistor.
 14. The integrated circuit of claim 13, wherein theat least one cascode transistor includes at least one N-type cascodetransistor and the bias circuit comprises: a second transistor and athird transistor electrically coupled to each other in a series fashion,wherein a gate and a drain of the third transistor are electricallycoupled with a node between the second and third transistors, and thenode between the second and third transistors is electrically coupledwith the well region of the at least one N-type cascode transistor. 15.The integrated circuit of claim 13, wherein the at least one cascodetransistor includes at least one P-type cascode transistor and the biascircuit comprises: a second transistor and a third transistorelectrically coupled to each other in a series fashion, wherein a gateand a drain of the second transistor are electrically coupled to eachother, and a node between the second and third transistors iselectrically coupled with the well region of the at least one P-typecascode transistor.
 16. The integrated circuit of claim 11, wherein theanalog circuit is an operational amplifier, a constant-gm bias circuit,a pump circuit, a converter, a multiplexer, or a bandgap referencecircuit.
 17. An integrated circuit comprising: a first P-type cascodetransistor electrically coupled with a first P-type transistor in aseries fashion; and a second P-type cascode transistor electricallycoupled with a second P-type transistor in a series fashion, whereingates of the first P-type cascode transistor, the second P-type cascodetransistor, the first P-type transistor, and the second P-typetransistor are electrically coupled to each other and with a drain ofthe first P-type cascode transistor.
 18. The integrated circuit of claim17 further comprising: a first N-type cascode transistor and a firstN-type transistor electrically coupled with the first P-type cascodetransistor and the first P-type transistor in a series fashion; a secondN-type cascode transistor and a second N-type transistor electricallycoupled with the second P-type cascode transistor and the second P-typetransistor in a series fashion; and a third P-type cascode transistor, athird P-type transistor, a third N-type cascode transistor, and a thirdN-type transistor electrically coupled in a series fashion, whereingates of the third P-type cascode transistor and the third P-typetransistor are electrically coupled to gates of the first P-type cascodetransistor, the second P-type cascode transistor, the first P-typetransistor, and the second P-type transistor, and wherein gates of thethird N-type cascode transistor and the third N-type transistor areelectrically coupled with gates of the first N-type cascode transistorand the second N-type cascode transistor and with a drain of the thirdN-type cascode transistor.
 19. The integrated circuit of claim 17further comprising: a first N-type cascode transistor, a first N-typetransistor, and a resistor electrically coupled with the first P-typecascode transistor and the first P-type transistor in a series fashion;and a second N-type cascode transistor and a second N-type transistorelectrically coupled with the second P-type cascode transistor and thesecond P-type transistor in a series fashion, wherein gates of the firstN-type cascode transistor, the second N-type cascode transistor, thefirst N-type transistor, and the second N-type transistor areelectrically coupled to each other and with a drain of the second N-typetransistor.
 20. The integrated circuit of claim 17 further comprising: afirst N-type cascode transistor, a first N-type transistor, a firstresistor, a second resistor, and a bipolar transistor electricallycoupled with the first P-type cascode transistor and the first P-typetransistor in a series fashion; and a second N-type cascode transistor,a second N-type transistor, a third resistor, and a second bipolartransistor electrically coupled with the second P-type cascodetransistor and the second P-type transistor in a series fashion, whereingates of the first N-type cascode transistor, the second N-type cascodetransistor, the first N-type transistor, and the second N-typetransistor are electrically coupled to each other and with a drain ofthe second N-type transistor.